Recoverable Versus Irrecoverable Fractions
“Agricultural Water Conservation in California with Emphasis on the San Joaquin Valley” (DH Report), by David C. Davenport and Robert M. Hagan, was published in 1982 by the Department of Land, Air, and Water Resources at the University of California at Davis, and their conclusions are largely true today, including:
It is erroneous to conclude that a particular irrigation system such as sprinkler or drip requires only a fraction of the water applied by systems such as furrow or border-strip…. Because of the recoverability and reusability of field runoff and deep percolation, it is even more erroneous to conclude that decreasing runoff and deep percolation will proportionally reduce the state’s net water deficit. Therefore, statements suggesting a 10-50% potential savings in agricultural water conservation by improving irrigation application systems are a disservice to the people of California because water policy and action programs based on such statements will substantially underestimate the state’s needs for future water supplies.
The main purpose of the DH Report and “Agricultural Water Use in California: A 2011 Update” is to explain the concept of recoverable versus irrecoverable fractions (note the DH Report used the term “losses” instead of “fractions”). When irrigation water is applied to a field to satisfy the needs of a crop, that water can end up in several different places, sometimes termed the destinations or fates of applied water. Many scientists discuss the issue using the term fractions, and this paper utilizes that terminology as well. These resulting fractions include:
- storage in the root zone for extraction by the crop to satisfy evapotranspiration demands
- immediate evaporation during the irrigation event
- immediate surface runoff during or soon after the irrigation event
- deep percolation—water infiltrating the soil, but moving below what is considered the effective root zone and not available for crop water use (see Additional Content: “Deep Percolation and Irrecoverable Fractions”)
- leaching for salt control—water that infiltrates the soil and then moves below the effective root zone and carries excess salts out of the root zone. Leaching is intentional deep percolation that is required to maintain root zone soil salinity below levels harmful to crop production or quality.
Different types of terminology can be used to further classify these fractions. They can be beneficial or non-beneficial fractions, and also consumptive or non-consumptive fractions. Beneficial refers to whether the fraction furthers the purpose of the water’s use, which in agriculture is to produce a profitable crop. For example, water stored in the root zone for use by the crop as a result of irrigation is a beneficial fraction. The term consumptive refers to whether the fraction is water that is left in the system (physically in or on the soil; in a useable aquifer; or in a canal, stream, river, etc.), or not. Examples of consumptive fractions are evapotranspiration by the crop and immediate evaporation during an irrigation event.
Soil water stored and then used for crop evapotranspiration is both a beneficial and a consumptive fraction (see Additional Content: “Key Definitions”). Deep percolation to maintain soil productivity that is beyond the required leaching fractions is considered a non-beneficial fraction but may be non-consumptive if the percolation moves to a useable aquifer.
Non-beneficial fractions at the field level are the result of irrigation inefficiencies, therefore an obvious goal for the irrigator is to minimize non-beneficial fractions. However, the key distinction is whether surface runoff or deep percolation resulting from irrigation on one field, which would be considered an inefficiency, can be recovered and reused on another field or farm, for M&I purposes, or for the environment. These recoverable fractions aren’t true depletions—rather, they are water that can be used at a different place and different time than the original diversion. In fact, recoverable fractions may be recovered and reused several times and possibly for different purposes—e.g., surface runoff from an irrigation event may support wetland habitats; deep percolation from an irrigation event can move to an aquifer and become an M&I water supply.
Irrecoverable fractions are the only true actual depletions resulting from inefficiencies (i.e., water volumes that are not available for reuse). These depletions occur when surface runoff or deep percolation resulting from irrigations move to another water body that is unusable for some reason. (It should be noted that during the runoff process there will be additional depletions such as free water surface evaporation and possibly consumptive uses by aquatic plants, streambank weeds, and trees; these are considered minor in the context of this discussion.)
The DH Report points out two facts that are central to the discussion of agricultural water use efficiency:
- Surface runoff, a non-beneficial fraction at the field level, may be captured and reused.
- Deep percolation above required leaching fractions, another type of non-beneficial fraction, may move to a useable aquifer.
Water moving off a field at the surface can be picked up, stored, and reused by agriculture, M&I, or the environment. In much the same manner, deep percolation from excessive irrigations (or just the need for leaching for salt control) can move to a usable aquifer where it can be reused through groundwater pumping. Many individual farms, as well as entire agricultural areas, rely on water that is pumped to the surface from an aquifer as their major source of water for the purpose of irrigation. Water percolates to an aquifer from natural rainfall, nearby rivers and streams, unlined canals, or from spreading basins. Deep percolation from inefficient irrigations, while perhaps a concern because of energy use and water quality, will also return water to the aquifer and be available for future groundwater pumping. A prime example of a large agricultural area such as this is the Salinas Valley where percolation from rainfall, the Salinas River, and streams from mountains on both sides of the valley, along with deep percolation from irrigation, is pumped (or re-pumped) to irrigate agricultural fields.
Non-beneficial fractions may also be irrecoverable and would be considered true depletions. Examples are:
- water that immediately evaporates during the irrigation event itself (Although this evaporation may reduce ETc during the irrigation, and this loss may be offset somewhat in certain situations.)
- surface runoff that goes to the ocean or other highly saline water body
- deep percolation that moves to an aquifer that is economically unusable due to poor water quality (salt sinks), or possibly even depth (due to the cost of pumping from that depth)
In summary, the only true depletions (losses) in terms of water volumes are irrecoverable fractions. All water users should continually strive to minimize irrecoverable fractions. However, recoverable fractions are just that, recoverable. There may be other undesirable impacts, and the range of uses (e.g., irrigation, recreation, human consumption, stock watering) may be diminished with each reuse and recovery, but, nonetheless, they are available.
Example 1 and 2 are intended to visually demonstrate the concepts of recoverable and irrecoverable fractions, and depict two major types of agricultural water use areas in California:
- Example 1- a flow-through system before and after on-farm irrigation efficiency improvements (Figures 1 and 2, Tables 1 and 2)
- Example 2- a closed-end system before and after on-farm irrigation efficiency improvements (Figures 3 and 4, Tables 3 and 4)
Some major options for improving on-farm irrigation efficiency include:
- Improving management of the individual irrigation events. This can occur with increased knowledge of the operating characteristics of the irrigation system or use of some for of irrigation scheduling in order to better match the irrigation to actual crop water needs.
- Improving maintenance of the system, especially micro and sprinkler systems, so the system operates at the intended efficiency
- Changing the irrigation system type to one better adapted to crop and field conditions. An example would be switching to a sprinkler or microsystem on a field that has two or more soil types making it difficult to irrigate uniformly with a flood-type system.
When examining these illustrations, the reader is alerted to the following:
- Each field will have a red, numbered arrow depicting its water supply (field inflows).
- Each field will have a blue, numbered arrow pointing up depicting the crop water use (crop evapotranspiration).
- Each field will have a dark blue, numbered arrow pointing down depicting the deep percolation from that field.
- Each field will have a yellow, numbered arrow at field bottom depicting surface runoff from the field.
- In many cases, the surface runoff from one field will be part of another field’s water supply.
- Values are for illustration purposes only. Assume they represent total acre-feet for a season.
Figures 1 and 2 are based on examples in the CALFED Record of Decision (CALFED 2006) and are representative of flow paths in the Sacramento Valley. Water is diverted from the river to various agricultural areas. There are few areas of underground or surface salt sinks in the valley; water that is diverted is largely consumed by crops or other plants, stored in the aquifer, or moves back to the river. A key assumption of the before and after scenarios depicted by Figures 1 and 2 is that the aquifer is kept in balance. There is the assumption in the before scenario of a movement of groundwater to the river (accretion), while in the after scenario it is assumed that the river recharges the aquifer to balance the increased well pumping and decreased deep percolation. (Tables 1 and 2 summarize the various destinations of diverted water and include summary statistics if the accretion/recharge is ignored.)
Examples of Recoverable Fractions. An example of a recoverable fraction in Figure 1 is the 40 units of runoff that flows off Field 1 and becomes Field 2’s entire irrigation supply before improvement of efficiencies. After improvement (see Figure 2), Field 2 receives only 20 units of water from Field 1 and now has to pump 15 units from the aquifer. The same situation occurs at Fields 6 and 7. Initially, Field 6 produced 40 units of runoff that was Field 7’s entire water supply. After improving on-farm efficiencies, Field 7 only receives 20 units of runoff from Field 6 and now pumps 15 units from the aquifer.
These situations can be characterized as shifting an indirect use of recoverable fractions to a (more manageable) direct use of controlled water supplies. The concept is that while Field 2 is getting an irrigation water supply from Field 1, the underlying assumption is that Field 2 could use that flow efficiently (in terms of irrigation efficiency or crop development) whenever it arrived from Field 1. This may require constructing and maintaining an on-farm reservoir at Field 2 to buffer the timing and amount of flows from Field 1. There may be water quality issues with the runoff to Field 2 (e.g., increased temperature, adsorbed chemicals on sediments, crop diseases, weed seeds). In the after scenario, 15 units of the 35-unit total supply is now pumped, and it is assumed on demand (i.e., whenever Field 2 needs it). The downside is the capital and operating costs of the well and the possibility of creating significant overdraft in the local aquifer. Note again the assumption that in the after condition, the river is recharging the aquifer by 53 units due to the increased pumping and the aquifer remains in balance.
Examples of Third-Party Impacts. There are also impacts to the small stream that captures runoff from Fields 2 and 3, the stream that captures runoff from Field 7, and some wildlife habitat at the bottom of Field 4. Before improving irrigation efficiency, there were up to 19 units of water flowing in the stream fed by Fields 2 and 3 supporting some habitat. Afterwards, this has been reduced by about 50%, and it is assumed the habitat will be adversely affected due to the decreased amount and quality of the water. The same effects can be seen at the bottom of Fields 4 and 7–as runoff is reduced, the habitat previously supported by it will be negatively impacted. One of the fields in this example is rice. Irrigated rice fields have been proven to provide much-needed waterfowl habitat (Hill 1999). If this crop was shifted to one with less seasonal ETc and/or no standing water in order to leave more water in the river, this habitat would be reduced or lost altogether.
Total Impacts of Improving On-Farm Efficiency. Example 1 demonstrates that improving on-farm irrigation efficiency may beneficially affect flows and quality in the river within the use area. It assumes the groundwater recharge from the river after on-farm efficiency improvements occurs at the end of the use area (Reach 7 is at 1,610 minus 53 units recharge equals 1,557 unit flow at the end, the same as the initial situation). What if the recharge occurs farther up the system–i.e., the recharge is continuous from the river throughout the use area? The increases in river flows noted in Table 1 would be lower while still resulting in impacts to other users.
Figures 3 and 4 resemble the CVP Friant system of the San Joaquin Valley floor adjacent to the southern Sierra mountains and foothills, where the Friant-Kern Canal is fed by Millerton Lake (created by Friant Dam), and in turn supplies many water districts along the canal as it winds south from Fresno into Kern County. The entire southern San Joaquin Valley (Tulare Basin) is a closed basin, and water does not move back to the San Joaquin River or out to the Pacific Ocean except in the wettest of years (a closed-end system).
As in Example 1, the after situation (Figure 4) indicates improved on-farm irrigation efficiency. In the example illustrated in Figures 3 and 4, the before and after situations demonstrate the following:
Improving on-farm efficiency may allow more irrigated acreage or more water-intensive crops to be grown Figure 4), as indicated by the higher ETcs on Fields 1, 3, and 4. There are water shortages (i.e., full contract amounts are not being delivered) in many agricultural areas south of the Delta and with increased on-farm efficiency some fallowed land could be brought back into production. An important point of this example is that the additional water within the system resulting from improved efficiencies is used to irrigate more crops. The recovered fractions are not left in the reservoir or sent outside the district. Note that this water could be sold and transferred to another user.
Improved On-Farm Efficiency Might Not Result in New Water Outside the Use Area.
Improved On-Farm Efficiency Can Create Third-Party Impacts. Improving on-farm efficiency can impact third parties as is noted by the change from a balanced aquifer to a 20-unit overdraft of the aquifer. (Note reduced recharge in Figure 4.) The city is completely dependent on groundwater for its supply. The overdraft could result in increased pumping costs to the city as groundwater levels decrease and could also threaten the long-term viability of the city’s water supply. It is important to see that the previous deep percolation fractions were being recovered for use by the city. Now that they are being recovered for use on the farms, the city’s water supply will be impacted. The improved on-farm efficiency did not create new water; it just changed the use of the affected water.
The overall impact on groundwater quality from reduced deep percolation is not clear. Less deep percolation could reduce the movement of nitrates and other soluble chemicals to the aquifer. If a salt balance is to be maintained in the soil to ensure crop production and quality, it may result in a higher salt concentration in the deep percolation that remains. This could eventually contribute to the overall salt concentration in the aquifer.
Improved On-Farm Efficiency Can Create New Water. This example also demonstrates that by reducing irrecoverable fractions, new water can be developed from improved on-farm efficiency. The deep percolation from Field 4 has been decreased by 10 water units that previously moved to an unusable salt sink.
Conjunctive Water Management. In Figures 3 and 4, there are three wells within the distribution area. When there are sufficient surface water supplies available from the Friant-Kern Canal, groundwater sources may not be used. However, in times of scarcity groundwater is used to augment, or even completely supplant, the canal supply. This is the concept behind conjunctive water management and water banks. In times of plenty water is transferred to dedicated recharge areas (note the percolation pond in Figures 3 and 4) so that the excess water is percolated and stored in the aquifer. This stored water is then used in times of drought. Intentional percolation using dedicated sites provides for high-quality water reaching the aquifer.
Examples 1 and 2 serve as bookends for the different types of conditions that exist both above and below the Sacramento-San Joaquin Delta. They are simplified versions of reality (e.g., depletions due to immediate evaporation or consumptive use by phreatophytes in the drainage waterways are ignored and there is no consideration of any deep percolation as required, and beneficial leaching fraction). Any number of examples could be constructed, and, indeed, any number of examples exist (e.g., a bathtub basin such as the Salinas Valley).
Both examples also identify an important challenge for water management in California–balancing the benefit/cost ratio to the agricultural enterprises, while providing benefits to the environment and addressing any third-party impacts. In Example 1, there are benefits to the river habitat and fishery from improved on-farm efficiency, but also an immediate third-party impact to some streamside habitat within the use area. There is also the possibility of loss of waterfowl habitat if cropping patterns or irrigation are significantly changed.
However, both examples illustrate the concept of recoverable fractions and their impact on irrigation efficiency when the spatial scale is expanded (i.e., instead of one field, we consider the entire use area). Looking at Table 1, it is seen that the individual field efficiencies range from 60-75% while the efficiency of the use area as a whole (assuming the aquifer is kept in balance) is 81%. In Table 2, the on-farm efficiencies have been improved to a range of 75-86%, but the use area efficiency is still about 80% (because of the assumption of recharge to aquifer from the river).
The illustrations in Figures 3 and 4 also demonstrate the concept. Table 3 indicates field efficiencies ranging from 70–78% but a use area efficiency of 80%. Improved efficiencies result in a range of 78-88% on-farm efficiency with a use area efficiency of 89%, also resulting in a 20-unit overdraft of the aquifer (Table 4).
Beyond the concept of recoverable versus irrecoverable fractions these examples identify the complex interactions that exist in environments developed over time. Understanding the concept of recoverable versus irrecoverable fractions means understanding that agricultural water users can be linked to M&I users and the environment in symbiotic relationships that cannot easily be undone. Typically, recoverable inefficiencies from irrigation events are recovered somewhere by someone in the system–by agriculture, M&I, or the environment–and are beneficially used.
Water use patterns in the California have developed over decades, especially those involving large storage/delivery projects, resulting in codependent partnerships. Careful analysis must be done to evaluate all impacts before simply calling for increased on-farm water use efficiency. Changes to these environments that result in perceived benefits to some users can also result in negative impacts to other third-party users. It is essential to identify and understand these consequences. As stated in Conclusion 1 from the DH Report (Davenport and Hagan 1982):
Within a crop season, water used in irrigation is either recoverable or is irrecoverably lost….if seepage, surface runoff, and deep percolation make contributions to soil moisture available to crops, groundwater, or wildlife habitat and recreation, that water cannot be regarded as lost. High priority should be given to preventing water flow to highly saline sinks…because inefficiencies are irrecoverable. However, conservation decisions must take into account environmental and instream needs as well as the appropriate balance of potential water savings against net farm income, possible reductions in food and fiber production, infrastructural viability, and the ability of farmers to retain flexibility in their operations and remain competitive in the market.
The complete Center for Irrigation Technology document can be downloaded at www.californiawater.org.